PARAMAGNETIC MEISSNER EFFECT IN NB

The paramagnetic Meissner effect (PME), or Wohlleben effect, in which the field-cooled magnetization of superconducting samples is paramagne tic below T-c, has been reported to occur in some samples of a variety of high-T-c cuprate superconductors. It has been proposed that the ef fect arose in granular hole-doped cuprates from current loops with pi phase shifts of the superconducting order parameter at some grain-boun dary junctions. It is argued that such behavior would be expected to o ccur in a d-wave superconductor, but not in a conventional s-wave supe rconductor. To test this hypothesis, we have searched for the occurren ce of the effect in Nb, and have confirmed a recent report by Minhaj e t al. of its occurrence in some Nb samples. For these studies, the eff ects of stray fields and field gradients in the measurement volume of the superconducting quantum interference device magnetometer have been carefully considered to rule out the possibility that measurement art ifacts might be responsible for the apparent paramagnetic behavior in Nb. The M(T) and M(H) curves obtained in Nb samples that show the PME also show remarkably strong resemblance to those curves reported for t he cuprate materials exhibiting the PME. Evidence is presented that th e effect arises from inhomogeneously trapped flux, and is strongly inf luenced by sample geometry and surface effects. These results suggest that, for the effect to be observable, T-c on the sample surface must be different from the bulk T-c. The occurrence of the PME in Nb strong ly suggests that the observation of this effect is unrelated to d-wave superconductivity.